Do my participants need to buy the phone app to sign up?

No! Participants/volunteers clicking a SignUp link from a smartphone will automatically be routed to a SignUp.com experience designed for small screens.

The iPhone and Android apps, "SignUp by SignUp.com" are pro-convenience apps giving organizers quick access to their SignUps and giving participants access to all their SignUps in one place, hours tracking, organizer messaging and more. Please see the iTunes app store or Android marketplace for more details.